Tube - Schrader 20"x1-3/8" (50) 40mm t is very important that you select the right size tubes for your rims. In most cases the required dimensions will be written on the side wall of your tyre for easy reference. It is also important to purchase tubes with valves that are compatible with your bike pump. There are two main types of valves used in bike tyre tubes: Schrader and Presta.